The invention disclosed and claimed herein generally pertains to an X-ray imaging system for enabling a system operator to quickly align an X-ray detector with the field of a beam projected by the system X-ray tube. More particularly, the invention pertains to a system of the above type which is provided with visual elements or indicators associated with the detector and the X-ray beam field, respectively, wherein the indicators are aligned with each other to ensure proper alignment of the detector and beam field. Even more particularly, the invention pertains to a system of the above type wherein the visual indicators are automatically adjusted to compensate for distortion resulting from X-ray beam angulation.
As is well known, in a typical X-ray imaging system a patient is positioned between an X-ray tube and an image receptor having a planar imaging surface, such as an X-ray film or a digital solid state detector. The tube projects a beam of X-radiation toward the detector surface and through body structure of the patient which is to be imaged. The area of projected X-radiation which is incident on the detector defines the active imaging area (AIA). Generally, the X-ray beam field or field of view (FOV), which is defined herein to be the intersection of the projected beam and the detector plane, must be coincident with, or lie within, the boundaries of the detector surface in order to avoid loss of image data. The FOV may be adjusted by rotating or tilting the tube to vary the direction of the projected X-ray beam, and also by operating a collimator to vary the width and length dimensions of the X-ray beam. Further adjustments may be made by linear translation of the tube and/or the detector.
In an important class of X-ray imaging systems, visual indicators are provided to enable a system operator or technician to quickly align the beam field and the detector, in an effort to provide the requisite coincidence therebetween. For example, in one product of such type a patient is horizontally supported upon a table, an X-ray tube is mounted above the table to project a beam downwardly, and a film cassette or other detector is located underneath the table. To assist the operator in aligning the detector and the X-ray beam, a mark or notch is formed in the detector handle, exactly at the mid-point of the planar detector surface along its length. Also, a beam of visible or laser light is projected downward from the tube, to provide a visible light field proximate to the detector plane. The light beam is optically guided or directed so that the boundaries of the light field substantially coincide with the boundaries of the X-ray beam field which will be projected by the tube for a particular tube orientation and collimator adjustment. Axes of the light field are identified by two thin lines of shadow, which are orthogonal to each other and intersect at a point defined by the intersection of the detector plane and the central axis of the projected beam. The shadow line axes also bisect the light field, along the mid-points of its length and width, respectively. Thus, even though the operator cannot easily view the detector, since it is positioned beneath the table, the operator can readily translate the detector to align the notch in the handle with the shadow line axis nominally located at the mid-point of the light beam field length.
If the tube is oriented so that the X-ray beam, or more particularly the central axis thereof, is directed in perpendicular or orthogonal relationship to the detector plane, the beam field projected into the detector plane will be of rectangular configuration. In this circumstance, the geometric center of the projected beam field will coincide with the point at which the central axis of the beam intersects the detector plane. Accordingly, the aforesaid shadow line axis will in fact be located at the mid-point of the beam field length. In this case, aligning the shadow line axis with the notch in the detector handle will effectively center the X-ray beam field along its length with the detector surface along its length, to provide the necessary coincidence therebetween.
However, an X-ray technician or operator, when setting up for an imaging procedure, may need to angulate the beam, that is, rotate or pivot the X-ray tube so that the beam is directed toward the detector at an angle of less than 90 degrees. This may be necessary, for example, to ensure that the beam passes through the specific body structure of the patient which is to be imaged. As the X-ray beam is increasingly angulated, the beam field projected into the detector becomes correspondingly distorted and trapezoidal, and the location of the point of intersection of the central beam axis becomes offset with respect to the geometric center of the projected X-ray field. As a consequence of these decentering and distorting effects, the conventional visual indicator arrangement described above will no longer align the center of the detector with the geometric center of the beam field, but rather with a point offset therefrom. This may cause anatomical cutoff to occur during the imaging process, whereby some of the image data acquired by the X-ray beam would not be received upon the detector. This, in turn, would necessitate that the examination be repeated, thus contributing to increased procedure cycle time, higher examination costs, and higher net radiation doses to the patient.
In an imaging system provided with a detector having a plane and also with an X-ray tube spaced apart from the plane, wherein the tube projects an X-ray beam into the plane to define a beam field therein, apparatus is provided for use by a system operator to selectively align the detector and the beam field. The apparatus comprises a first viewable element which indicates the position of the detector along a reference axis lying in the detector plane, and further comprises a computational device for producing a signal representing an offset along the reference axis, between the geometric center of the beam field and the point at which the central axis of the projected X-ray beam intersects the detector plane. Structure is provided to respectively support the tube and the detector to enable relative translational movement therebetween along the reference axis. An indicator device, responsive to the produced signal, provides notice that the first element is positioned in a pre-specified relationship with the beam field geometric center.
In a preferred embodiment of the invention, the detector is provided with an X-ray detection surface lying in the detector plane, the surface having a length dimension which extends along the reference axis, and the first element comprises a visually observable element positioned at the center of the length dimension of the detector. The indicator device provides visual or viewable notice when the first element and the beam field geometric center are in alignment along the reference axis. Preferably also, the computational device is disposed to compute the offset as a function of the spacing between the tube and the detector plane, and of the angles respectively characterizing the direction and width of the projected beam.
In a useful embodiment, the indicator device comprises a linear array of light emitting diodes (LED""s), or of other light emitting elements as defined hereinafter, which extends along the reference axis in parallel relationship therewith. A given one of the light elements is illuminated by the produced signal representing offset, and is available for use by an operator to align the first viewable element, and therefore the center of the detector, with the geometric center of the beam field.
In another useful embodiment, the apparatus is further provided with a second element indicating the position of the beam field axis intersection point, and with a tracking device disposed to monitor the linear travel of the detector, along the reference axis, from an initial position at which the first and second elements are in alignment.